U.S. patent number 6,534,438 [Application Number 09/626,156] was granted by the patent office on 2003-03-18 for catalyst composition.
This patent grant is currently assigned to BP Chemicals Limited, Johnson Matthey Public Limited Co.. Invention is credited to Michael James Baker, John William Couves, Kenneth George Griffin, Peter Johnston, James Colin McNicol, George Frederick Salem.
United States Patent |
6,534,438 |
Baker , et al. |
March 18, 2003 |
Catalyst composition
Abstract
Composition and process for making same in which the composition
includes support particles having at least one catalytically active
metal or precursor thereof distributed therein in a layer below the
surface of the particle. The layer is located between an inner and
an outer region of the support particle, and each of the inner and
outer regions has a lower concentration of the metal or precursor
thereof than the layer.
Inventors: |
Baker; Michael James (Feltham,
GB), Couves; John William (High Wycombe,
GB), Griffin; Kenneth George (Royston, GB),
Johnston; Peter (Royston, GB), McNicol; James
Colin (Pinner, GB), Salem; George Frederick
(Naperville, IL) |
Assignee: |
BP Chemicals Limited (London,
GB)
Johnson Matthey Public Limited Co. (London,
GB)
|
Family
ID: |
24509183 |
Appl.
No.: |
09/626,156 |
Filed: |
July 26, 2000 |
Current U.S.
Class: |
502/325;
252/520.3; 502/327; 502/328; 502/330; 502/331; 502/332; 502/333;
502/339; 502/527.12; 502/527.13 |
Current CPC
Class: |
B01J
35/08 (20130101); C07C 67/055 (20130101); B01J
35/0006 (20130101); B01J 37/0201 (20130101); C07C
67/055 (20130101); C07C 69/01 (20130101); C07C
67/055 (20130101); C07C 69/15 (20130101) |
Current International
Class: |
B01J
37/02 (20060101); B01J 37/00 (20060101); B01J
35/00 (20060101); B01J 35/08 (20060101); C07C
67/00 (20060101); C07C 67/055 (20060101); B01J
023/00 (); B01J 023/40 (); B01J 023/42 (); B01J
023/58 (); B01J 023/72 () |
Field of
Search: |
;502/325,327,328,329,330,331,332,333,339,527.12,527.13,527.15
;252/520.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0 569 624 |
|
Nov 1993 |
|
EP |
|
1 500 167 |
|
Feb 1978 |
|
GB |
|
1 521 652 |
|
Aug 1978 |
|
GB |
|
WO 97/36678 |
|
Oct 1997 |
|
WO |
|
99/62632 |
|
Dec 1999 |
|
WO |
|
Primary Examiner: Silverman; Stanley S.
Assistant Examiner: Nguyen; Cam N.
Attorney, Agent or Firm: Nixon & Vanderhye
Claims
We claim:
1. A composition comprising support particles having at least one
catalytically active metal or precursor thereof distributed
therein, in which the metal or precursor thereof is distributed in
the support particle in a layer below the surface of said particle,
said layer being between an inner and an outer region of said
support particle, and each of said inner and outer regions having a
lower concentration of said metal or precursor thereof than said
layer, said layer having an outer edge which is at least 3 microns
and no more than 20 microns below the surface of each support
particle.
2. A composition as claimed in claim 1 in which the layer
containing the catalytically active metal or precursor thereof has
an outer edge which is 4 to 20 microns below the surface of each
support particle.
3. A composition as claimed in claim 2 in which the layer
containing the catalytically active metal or precursor thereof has
an outer edge which is 5 to 15 microns below the surface of each
support particle.
4. A composition as claimed in claim 1 in which the layer has an
average thickness which is less than half the radius of the
particle.
5. A composition as claimed in claim 1 in which the layer has an
average thickness of greater than 0.1 microns.
6. A composition as claimed in claim 5 in which the layer has an
average thickness of greater than 0.1 microns and less than 25
microns.
7. A composition as claimed in claim 5 in which at least 80% of the
support particles have a mean diameter of less than 300
microns.
8. A composition as claimed in claim 1 in which at least 80% of the
support particles have a mean diameter of less than 300
microns.
9. A composition as claimed in claim 8 in which at least 90% of the
support particles have a mean diameter of less than 300
microns.
10. A composition as claimed in claim 1 in which the catalytically
active metal comprises at least one Group VIII noble metal.
11. A composition as claimed in claim 10 comprising palladium; at
least one promoter selected from the group consisting of gold,
copper, cerium and mixtures thereof; and at least one other
promoter selected from the group consisting of salts of Group I,
Group II, lanthanide and transition metals.
12. A composition as claimed in claim 11 wherein said inner and
outer regions have a lower concentration of said promoter metal
selected from the group consisting of gold, copper, cerium and
mixtures thereof than said layer.
13. A composition as claimed in claim 12 comprising palladium, gold
and potassium acetate.
14. A composition as claimed in claim 11 comprising palladium, gold
and potassium acetate.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a metal catalyst composition and
in particular to a supported metal catalyst composition. The
present invention also relates to a process for making such a
catalyst composition
Supported metal catalysts are typically made by impregnating a
suitable support material with a catalytically active metal or with
its precursor. For example, catalysts for use in the production
vinyl acetate monomer (VAM) by the reaction of ethylene, acetic
acid and oxygen are made by impregnating a support such as silica
or alumina with a compound of a Group VIII noble metal such as
palladium together with a gold compound and an alkali metal salt,
typically in the form of an acetate, the palladium and gold
compounds being converted to catalytically active state.
In early examples of fixed-bed catalysts for use in the production
of VAM, palladium and gold were distributed more or less uniformly
throughout the support, for example, U.S. Pat. No. 3,743,607. Since
gaseous reactants do not diffuse significantly into the large
fixed-bed catalyst particles, much of the expensive catalytic metal
components in the interior of the catalyst were not useful.
Subsequently, shell-impregnated, fixed-bed catalysts were developed
in which most of the catalytic metals were deposited onto an outer
shell of the support particle. For example, Great Britain Patent
No. 1,500,167 describes a catalyst in which at least ninety percent
of the palladium and gold is distributed in that pant of the
support particle which is not more than thirty percent of the
particle radius from the surface. The palladium and gold being
at/near the surface are susceptible to loss through attrition.
In the preparation of shell-impregnated, fixed bed catalysts such
as that described in GB 1,500,167 and EP-A-0 569 624, after
impregnation of a support with a Group VIII noble metal solution,
the noble metal is subsequently precipitated to the support by, for
example, treatment with an aqueous solution of an alkali metal
salt. Such precipitated noble metal has limited mobility.
U.S. Pat. No. 4,677,084 describes a process for preparing attrition
resistant catalyst, catalyst precursor and catalyst support
particles and in particular silica-containing vanadium/phosphorus
oxide catalysts. The catalyst, catalyst precursor or catalyst
support particles are slurried in a solution of an oxide such as
silica. The slurry is then spray-dried and calcined to produce
microspheres. The process results in the formation of an oxide-rich
layer at the periphery of each calcined microsphere.
There remains a need for an improved metal catalyst composition and
in particular, a supported metal catalyst composition.
BRIEF DESCRIPTION OF THE INVENTION
Thus, according to the present invention there is provided a
catalyst composition comprising support particles having at least
one catalytically active metal distributed therein, in which the
metal is distributed in the support particle in a layer below the
surface of said particle, said layer being between an inner and an
outer region of said support particle, and each of said inner and
outer regions having a lower concentration of said metal than said
layer.
The catalyst composition can provide high attrition resistance as
well as high activity. The outer region of the catalyst composition
may also provide some resistance to poisoning of the catalytically
active metal.
The present invention also provides a process for preparing a
supported metal catalyst composition which process comprises
impregnating support particles with a solution of least one
catalytically active metal, or precursor thereof, such that the
metal, or its precursor, is in a mobile state in the support
particles and then treating the mobile metal, or precursor, in the
support particles with at least one chemical reagent to deposit and
immobilize the metal, or its precursor, in the support particles
such that the metal, or its precursor, is distributed in the
support particle in a layer below the surface of said support
particle, said layer being between an inner and an outer region,
each of said inner and outer regions having a lower concentration
of said metal or precursor than said layer.
Also, according to the present invention there is provided a
composition comprising support particles having at least one
precursor of a catalytically active metal distributed therein, in
which the precursor is distributed in the support particle in a
layer below the surface of said particle, said layer being between
an inner and an outer region of said support particle, and each of
said inner and outer regions having a lower concentration of said
precursor than said layer.
An advantage of the process of the present invention is that by
treating a catalytically active metal, or its precursor, which is
in a mobile state in the support particle with at least one
chemical reagent which deposits and immobilizes it, the metal, or
its precursor, is distributed predominantly in a layer below the
surface of the particle such that the catalyst composition so
produced has high attrition resistance as well as high
activity.
Preferably, the concentration of catalytically active metal or of
its precursor in each of the inner and outer regions is less than
half the concentration of the catalytically active metal or of its
precursor in the layer.
In a preferred embodiment, the layer containing the catalytically
active metal, or its precursor, has an outer edge which is at least
3% and no more than 75% of the particle radius from the surface of
the support particle and preferably, at least 5%, and more
preferably at least 10% of the particle radius from the surface of
the support particle.
Depending upon the size of the support particles, alternatively or
additionally, the layer containing the catalytically active metal,
or its precursor, preferably has an outer edge which is at least 3
microns and no more than 20 microns below the surface of each
support particle, and is more preferably 4 to 20 microns below the
surface of each particle, and yet more preferably is 5 to 15
microns below the surface of each particle.
Typically, the layer has an average thickness which is less than
half the radius of the particle, for example less than 25 microns.
Preferably, the layer has an average thickness of greater than 0.1
microns.
The process for preparing the catalyst composition of the present
invention may be used for the preparation of catalysts for use in
fixed bed or preferably, fluid bed processes, for example, for the
production of vinyl acetate monomer.
A suitable support material for use in a fluid bed process is a
microspheroidal particulate material. When the catalyst composition
is to be used in a fluid bed process, as is well known in the fluid
bed art, the support particles must be small enough to be
maintained in a fluid bed state under reaction conditions while
keeping sufficient attrition resistance such that excessive amounts
of catalyst composition need not be replenished during the process.
Further, although typical particle sizes (as measured by mean
particle diameters) should not be so large as to be difficult to
keep in a fluid bed state, there should not be an excessive amount
of very small particles (fines) which are difficult to remove from
the system and may plug gas recycle lines. Thus, typically suitable
fluid bed support particles have a distribution of larger to
smaller particle sizes.
For example, in the fluid bed manufacture of vinyl acetate from
ethylene, acetic acid and oxygen-containing gas, typically, at
least 80% and preferably at least 90% of the support particles have
mean diameters of less than about 300 microns.
A typical catalyst useful in this invention may have the following
particle size distribution:
0 to 20 microns 0-30 wt % 0 to 44 microns 0-60 wt % 44 to 88
microns 10-80 wt % 88 to 106 microns 0-80 wt % >106 microns 0-40
wt % >300 microns 0-5 wt %
Persons skilled in the art will recognize that support particles
sizes of 44, 88, and 300 microns are arbitrary measures in that
they are based on standard sieve sizes. Particle sizes and particle
size distributions may be measured by an automated laser device
such as a Microtrac X100.
Microspheroidal support particles useful in the present invention
are sufficiently porous to permit gaseous reactants to diffuse into
the particle and contact catalytic sites incorporated within the
particle. Thus, the pore volume should be high enough to permit
gaseous diffusion. However, a support particle with an exceedingly
high pore volume typically will not have sufficient attrition
resistance or will not have sufficient surface area for catalytic
activity. A typically suitable microspheroidal support particle has
a pore volume (measured by nitrogen sorption) between about 0.2 and
0.7 cc/g. A preferable support particle has a pore volume between
about 0.3 and 0.65 cc/g and more preferably between about 0.4 and
0.55 cc/g.
Surface areas (measured by nitrogen BET) for fluid bed support
particles with mean diameters and pore volumes useful in the
present invention typically are above about 50 m.sup.2 /g and may
range up to about 200 m.sup.2 /g. A typical measured surface area
is about 60 to about 125 m.sup.2 /g.
Typically useful support particles, especially silica support
particles are described in U.S. Pat. No. 5,591,688, incorporated by
reference herein. In these supports microspheroidal particles are
produced by spray drying a mixture of a silica sol with silica
particles followed by drying and calcining. In the preparation, at
least 10 wt. %, preferably at least 50 wt. %, of a silica so is
mixed with particulate silica. A useful particulate silica is a
filmed silica such as Aerosil.RTM. (Degussa Chemical Company). A
typical silica particulate material has a high surface area (about
200 m.sup.2 /g) with essentially no micropores, and, typically, are
aggregates (with mean diameters of several hundred nm) of
individual particles with average diameters of about 10 nm (above 7
nm). Preferably, the silica is sodium free. Sufficient particulate
silica is added to the mixture to obtain a desired pore volume in
the resulting support particle. The amount of particulate silica
may range up to 90 wt. % and typically ranges up to 10 to 50 wt. %
of the silica in the mixture. Typically, the silica sol/particulate
silica mixture is spray dried at an elevated temperature such as
between 115.degree. to 280.degree. C., preferably 130.degree. to
240.degree. C., followed by calcining at temperature typically
ranging from between 550.degree. to 700.degree. and, preferably
630.degree. to 660.degree. C.
An advantageous silica sol for preparing a catalyst support useful
in the present invention contains silica particles in the sot
typically more than 20 nanometers in mean diameter and may be up to
about 100 nanometers or more. Preferable sols contain silica
particles of about 40 to 80 nanometers. Nalco silica sol 1060
particularly is advantageous because of the relatively large mean
silica particle sizes of 60 nm pack less efficiently than smaller
sol particles such as Nalco 2327 at about 20 nm. The larger
particle size sol yields a final support with higher mesopore
volume and less micropore volume.
A suitable support material for use in a fixed bed vinyl acetate
process may be spherical. The support particles typically may have
a diameter of 3 to 9 mm. Surface areas (measured by nitrogen BET)
for fixed bed support particles with mean diameters and pore
volumes useful in this invention typically are above about 5
m.sup.2 /g and may range up to about 800 m.sup.2 /g. Suitably, a
fixed bed support particle has a pore volume (measured by nitrogen
sorption) between about 0.2 and 3.5 ml/gram. Suitably, fixed bed
support particles have an apparent bulk density of 0.3 to 1.5
gram/ml.
Although silica-based support particles are the most preferred in
this invention for use in fluid or fixed bed processes, other
oxides may be used as long as a particle of appropriate size and
with sufficient pore volume is produced in which may be deposited
the required catalytic materials. Possible other oxides include
alumina, silica-alumina, ceria, magnesia, titania, zirconia and
mixed oxides and mixtures thereof. The support may be impregnated
with organic or inorganic bases for example Group I or Group II
hydroxides and ammonium hydroxide.
Preferably, the catalytically active metal comprises at least one
Group VIII noble metal. The noble metals of Group VIII of the
Periodic Table of the Elements (IUPAC) are palladium, platinum,
rhodium, ruthenium, osmium and iridium. Typically, the noble metal
used in a catalyst composition for the manufacture of vinyl acetate
comprises palladium. Such a catalyst composition typically contains
at least about 0.1%, preferably at least 0.2 wt % palladium to
about 5 wt % and preferably up to 4 wt % palladium.
The catalytically active metal(s) may be impregnated in one or more
steps onto the support particles in the form of precursor salt
solutions. In a preferred aspect of the present invention,
microspheroidal support particles are preferably impregnated with a
palladium compound in a suitable solvent. Suitable solvents may be
water, carboxylic acids such as acetic acid, benzene, toluene,
alcohols such as methanol or ethanol, nitriles such as acetonitrile
or benzonitrile, tetrahydrofuran or chlorinated solvents such as
dichloromethane. Preferably, the solvent is water and/or acetic
acid. Suitably, the support particles are impregnated with
palladium acetate, sulphate, nitrate, chloride or
halogen-containing palladium compounds such as H.sub.2 PdCl.sub.4,
which is sometimes also represented as [PdCl.sub.2 ]2HCl, and Group
I or Group II salts thereof such as Na.sub.2 PdCl.sub.4 and K.sub.2
PdCl.sub.4. A preferred water soluble compound is Na.sub.2
PdCl.sub.4. A preferred acetic acid-soluble palladium compound is
palladium acetate.
The catalyst composition suitable for the manufacture of vinyl
acetate may also comprise, as promoters, other metals such as gold,
copper, cerium and mixtures thereof, preferably gold. These other
metals may also be more concentrated in the layer than in the inner
and outer regions, that is the inner and outer regions may have a
lower concentration of said promoter metal than said layer.
Typically, the weight percent of gold is at least about 0 1 wt %,
preferably, at least 0.2 wt % gold to about 3 wt % and S preferably
up to 1 wt % gold. Typically, the weight percent of cerium is at
least about 0.1 wt %, preferably at least 0.2 wt % to about 10 wt %
or more, preferably up to 5 wt % of cerium. Typically, the weight
percent of copper is at least 0.1 to about 10 wt %, preferably up
to 5 wt % copper.
Impregnation of the support particles with the gold, copper, cerium
or mixtures thereof may be carried out together with or separately
from the impregnation of the support particles with the Group VIII
noble metal compounds such as palladium compound(s). Suitable gold
compounds include gold chloride, dimethyl gold acetate, barium
acetoaurate, gold acetate, tetrachloroauric acid (HAuCl.sub.4,
sometimes represented as AuCl.sub.3 HCl) and Group I and Group II
salts of tetrachloroauric acid such as NaAuCl.sub.4 and
KAuCl.sub.4. Preferably, the gold compound is HAuCl.sub.4. The gold
compounds may be prepared in situ from suitable reagents. These
promoters may be used in an amount of 0.1 to 10% by weight of each
promoter metal present in the finished catalyst composition.
In catalyst compositions suitable for the production of vinyl
acetate, in addition to Group VIII noble metals such as palladium
and optional promoter selected from gold, copper and cerium, the
support particles may also be impregnated at any suitable stage
during the preparation process with one or more salts of Group I,
Group II, lanthanide and transition metals promoters, preferably of
cadmium, barium, potassium, sodium, manganese, antimony, lanthanum
or mixtures thereof, which are present in the finished catalyst
composition as salts, typically acetates. Generally, potassium will
be present. Suitable salts of these compounds are acetates but any
soluble salt may be used. These promoters may be used in an amount
of 0.1 to 15%, preferably 3 to 9%, by weight of each promoter salt
present in the finished catalyst composition.
The impregnation of the support particles may be performed using
any suitable technique. A preferable method to impregnate salt
solutions is an incipient wetness technique in which there is used
a salt solution in an amount up to the volume of the pores of the
support particles without excess solution being used. Thus, a
desired level of metal compounds such as palladium and other metal
species may be incorporated into the support particles by
calculating the amount of metals and the volume of solution needed.
The impregnation is typically performed at ambient temperature.
Elevated temperatures may be used for example, with palladium
acetate in acetic acid, greater than 60.degree. C. and up to
120.degree. C.
The impregnated support particles may optionally be dried and the
impregnation step repeated two or more times if there is required
higher metal or promoter loadings, than the solubility of the salt
in the solvent will allow. The drying step may be performed at up
to 140.degree. C., preferably up to 120.degree. C. The drying step
may be performed at ambient temperature and reduced pressure. Air,
nitrogen, helium, carbon dioxide or any suitable inert gas may be
used in the drying step. The catalyst composition may be tumbled,
rotated or agitated by the gas stream or mechanical means to aid
drying.
The impregnated support particles are preferably washed to remove
anion contaminants, for example, nitrates, sulphates and usually
halides. For chloride removal, washing with de-ionised water should
proceed until a silver nitrate test shows that there is no soluble
chloride present. The anion contamination levels should be
minimised for the preparation of catalyst compositions suitable for
the production of vinyl acetate. Cation contaminants should be
minimised for the preparation of catalyst compositions for the
production of vinyl acetate; for example to below 0.5 wt %,
preferably below 0.2 wt % of sodium in the dried catalyst
composition. Low levels of these contaminants are likely to remain;
it is not essential that the levels are absolutely zero. On a
commercial scale, batch washing may be used To speed up the
process, warm water may be used Also, ion exchange solutions (such
as potassium acetate) can be used to displace chloride and sodium.
Also, the reagents used for the preparation can be selected to
avoid the use of chloride and sodium, for example, potassium
metasilicate instead of other Group I or Group II salts such as a
sodium salt.
The support may be impregnated with base.
The chemical reagent used to deposit and immobilize the metal or
its precursor may be added to the metal- or precursor-impregnated
support particles before or after the optional washing step,
depending on the reagents used. The chemical reagent may be a
reducing agent such as hydrazine, formaldehyde, sodium formate,
sodium borohydride, methanol or alcohols, preferably hydrazine.
Hydrazine is preferably used as an aqueous solution. Gaseous
reagents for example, hydrogen or hydrocarbons such as ethylene,
may be used as alternative chemical reagents to deposit and
immobilize the metal or its precursor.
It has been found that the amount of chemical reagent needed to
give a layered structure depends on the amount of metal e.g. Group
VIII noble metal such as palladium metal which is present in the
catalyst composition. Generally, higher concentrations of chemical
reagent than have hitherto been used are used. Thus, for example,
if palladium is present in the catalyst composition in amounts of
0.5-2 wt %, a hydrarine concentration in water in excess of 2 wt %,
for example, at least 3 wt %, such as 4-20 wt % and preferably 4-8
wt % has been found to produce a layered structure of the present
invention. It has also been found that the more concentrated the
solution of hydrazine the greater the distance the layer will be
below the surface of the particle.
Typically, the chemical reagents such as aqueous hydrazine are used
at ambient temperatures but temperatures up to 100.degree. C. may
be used. Typically, an excess of chemical reagent is used. The
reagent will reduce the impregnated metal precursor species to
catalytically active zero valence noble metal crystallites.
Preferably hydrazine at a concentration in water of at least 2 wt
%, preferably in excess of 2 wt %, for example at least 4 wt % is
used in the preparation of the catalyst compositions. Thus,
according to another embodiment of the present invention there is
provided a process for preparing a catalyst composition wherein
said process comprises impregnating support particles with a
solution of least one Group VIII noble metal and then contacting
the impregnated support with hydrazine at a concentration in water
of at least 2 wt %.
Contacting the chemical reagent with the mobile metal- or
precursor-impregnated support particles deposits and immobilizes
the metal or its precursor such that the metal or its precursor is
distributed as a layer below the surface of each support particle.
Preferably, at least 50% of the metal is distributed as a layer in
each support particle. The distribution of the metal may be
determined by suitable techniques such as Electron Microscopy.
Base may be used to influence the mobility of the metal or its
precursor and to affect the size and location of the layer. The
base may be added before or during the impregnation of the support
with the metal or its precursor. There should not be used so much
base as to completely immobilize the metal or its precursor before
addition of the chemical reagent.
The catalyst compositions of the present invention may be used in
fixed or fluid bed reactors for the production of vinyl acetate, by
the reaction of ethylene and acetic acid with molecular oxygen
containing gas in the presence of the catalyst composition.
Preferably, a fluid bed reactor is used to produce vinyl acetate
under fluidised bed reaction conditions. The reaction temperature
suitably is maintained at about 100.degree. to 250.degree. C.,
preferably 130.degree. to 190.degree. C. The reaction pressure
suitably is about 50 to 200 psig (3 to 14 barg), preferably 75 to
150 psig (5 to 10 barg). In a fluid bed reactor system, the
particles of the catalyst composition are maintained in a fluidized
state by sufficient gas flow through the system. This gas flow
preferably is maintained at a suitable level to maintain the
fluidization. Excess flow rate may cause channeling of the gas
through the reactor which decreases conversion efficiency.
Additional alkali metal salt promoter may be added during the
process to maintain activity.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be illustrated by reference to the following
examples and drawings in which FIG. 1 illustrates a cross-section
of a typical catalyst particle according to the present invention
and FIG. 2 illustrates an X-ray profile through a section of a
catalyst particle according to the present invention.
In FIG. 1, a catalyst particle (1) is provided with a layer (2) of
a Group VIII noble metal such as palladium. The layer (2) may
optionally contain other metals such as gold. The layer (2) is
located with an outer edge (7) below the particle surface (3) and
between an inner region (4) and an outer region (5) of the catalyst
particle having lower concentrations of the Group VIII noble metal
and/or other metals than the layer (2).
FIG. 2 illustrates an X-ray profile from beyond and across a
section of a catalyst particle containing palladium and gold using
an Electron Microprobe Analyser, for example along line X-X' of
FIG. 1. FIG. 2 clearly illustrates that the palladium and gold are
both mainly distributed in two specific locations along the
diameter of the particle (that is, as a layer below the surface of
the particle) with lower concentrations of palladium and gold
distributed elsewhere. In FIG. 2 the outer edges of the particle
are at positions labeled A and A' at 0 and 97 microns.
EXAMPLES
The following Examples illustrate but do not limit the invention
described and claimed herein.
Preparation of the Support Particles
Preformed microspheroidal support particles comprising 100% silica
were used for all the Examples described below.
The support particles were prepared by spray-drying a mixture of
Nalco silica sol 1060 (Nalco Chemical Company) and Degussa
Aerosil.RTM. silica. (Degussa Chemical Company). In the dried
support particles, 80% of the silica came from the sol and 20% of
the silica came from the Aerosil. The spray-dried microspheroidal
support particles were calcined in air at 640.degree. C. for 4
hours. Prior to use the support particles were sieved and a
specific particle size distribution was used in the preparation of
the catalyst compositions as follows:
Particle size % >300 microns 2 88-300 microns 30 44-88 microns
38 <44 microns 30
Comparative Example 1 and Examples 2, 3 and 4
The following procedure describes the method of preparation of four
fluid bed vinyl acetate catalyst compositions (1.6 wt % palladium,
0.7 wt % gold, 7 wt % potassium acetate on silica).
Preparation of Catalyst Compositions
Silica support particles (163g) were impregnated with a solution of
Na.sub.2 PdCl.sub.4.xH.sub.2 O (containing 2.9 g palladium) and
HAuCl.sub.4.xH.sub.2 O (containing 1.2 g gold) in distilled water
by incipient wetness. The resulting mixture was mixed thoroughly,
left to stand for 1 hour and dried overnight.
A 35 g portion of the dried impregnated material was added slowly
to each of a 1 wt % (Comparative Example 1), 2 wt %, 4 wt %/, and 8
wt % (Examples 2-4) solution of hydrazine in distilled water at
room temperature and the mixture was allowed to stand with
occasional stirring. Thereafter the mixture was filtered, washed
with distilled water and dried overnight.
The material was then impregnated with an aqueous solution of
potassium acetate (2.6 g) by incipient wetness. The resulting
mixture was mixed thoroughly, left to stand for 1 hour and dried
overnight.
Comparative Example 5 and Example 6
The following procedure describes the method of preparation of
fluid bed vinyl acetate catalyst compositions (1.6 wt % palladium,
0.7 wt % gold, 7 wt % potassium acetate on silica).
Preparation of Catalyst Compositions
Silica support particles (163 g) were impregnated with a solution
of Na.sub.2 PdCl.sub.4.xH.sub.2 O (containing 2.9 g palladium) and
HAuCl.sub.4.xH.sub.2 O (containing 1.2 g gold) in distilled water
by incipient wetness. The resulting mixture was mixed thoroughly,
left to stand for 1 hour and dried overnight.
A 35 g portion of the dried impregnated material was added slowly
to each of a 1 wt % (Comparative Example 5) and 8 wt % (Example 6)
solution of hydrazine in distilled water at 80.degree. C. and the
mixture was allowed to stand with occasional stirring. Thereafter
the mixture was filtered, washed with distilled water and dried
overnight.
The material was then impregnated with an aqueous solution of
potassium acetate (2.6 g) by incipient wetness. The resulting
mixture was mixed thoroughly, left to stand for 1 hour and dried
overnight.
Example 7
The following procedure describes the method of preparation of a
fluid bed vinyl acetate catalyst composition (1.0 wt % palladium,
0.4 wt % gold, 5 wt % potassium acetate on silica).
Preparation of Catalyst Composition
Silica support particles (468 parts by weight) were impregnated
with a solution of Na.sub.2 PdCl.sub.4.xH.sub.2 O (containing 5.00
parts by weight palladium) and HAuCl.sub.4.xH.sub.2 O (containing
2.00 parts by weight gold) in distilled water by incipient wetness.
The resulting mixture was mixed thoroughly and dried overnight.
The dried impregnated material was added slowly to a room
temperature solution of hydrazine in distilled water (4 wt %), and
the mixture was allowed to stand with occasional stirring.
Thereafter the mixture was filtered, washed with distilled
water.
The material was doped with solid potassium acetate (25.0 parts by
weight). The resulting mixture was mixed thoroughly and dried
overnight.
Measurement of Noble Metal Layer Depth
A sample (approx, 20 particles) from each of Comparative Examples 1
and 5 and examples 2-4 and 6-7 was set in Araldite.RTM. resin
overnight at 60.degree. C. Thin sections (<100 nm) were cut with
a diamond knife from prepared `mesas`. The images were recorded by
Transmission Electron Microscopy (TEM) using a JEOL 2000FX
instrument. Photographic plates of the catalyst composition
particles were then studied and measurements of the depth of the
major layer of metals from the particle surface were made. No
measurements were made on any particle with a diameter of 25
microns or less. The results are given in Table 1.
TABLE 1 Average layer Average Hydrazine depth below layer
concentration Reduction surface thickness Example (wt %)
temperature (microns) (microns) Comparative 1 1 room temp. 1 1 2 2
room temp. 3 0.75 3 4 room temp. 6 0.25 4 8 room temp. 8 0.25
Comparative 5 1 80.degree. C. 1 1.5 6 8 80.degree. C. 10 0.5 7 4
room temp. 8 0.25
The data illustrates that as the hydrazine concentration increases
the layer depth below the surface increases. This effect is
observed at both room temperature and at 80.degree. C. The greater
the layer depth the more protected the noble metal catalyst
component will be from loss by abrasion or attrition.
Example 8 - Preparation of Catalyst Composition Without Gold
Silica support (47 g) was impregnated with a solution of Na.sub.2
Pd.sub.2 Cl.sub.4.xH.sub.2 O (containing 0.5 g palladium) in
distilled water by incipient wetness. The resulting material was
mixed thoroughly and thereafter dried overnight.
The dried impregnated material was added slowly to a stirred
solution of hydrazine in distilled water (4 wt. %) and the mixture
allowed to stand with occasional stirring. Thereafter the material
was filtered and washed with distilled water.
The material was impregnated with an aqueous solution of potassium
acetate (2.5 g) by incipient wetness. The resultant material was
left to stand for 1 hour and dried overnight.
Examination of this material showed that it also had a layered
structure but that the layer of palladium was broader than in the
catalyst materials of examples 2-4 and 6-7.
Metal Loss Experiment
Microspheroidal catalysts containing palladium and gold were
subjected to attrition tests in a 38 mm internal diameter fluid bed
test apparatus provided with a freeboard section and air feed
through three 0.4 mm diameter nozzles with a gas velocity of 320
m/s. 50 g samples of catalyst were used in 20 hour tests which were
designed to mimic attrition in a fluid bed reactor for the
production of vinyl acetate from ethylene, acetic acid and oxygen,
but under accelerated conditions. The freeboard section of the
apparatus enabled the bulk of the catalyst to be retained in the
vessel during the experiment, but fines formed by attrition escaped
from the top of the vessel and were collected in filters and
measured. The metal content of the recovered fines was measured and
expressed as a percentage of the metal in the catalyst. This
provided a measure of the attrition.
Catalyst A was a shell type catalyst whereas catalyst B had been
prepared by a process according to the present invention. A
significant proportion of the palladium and gold in catalyst B was
located in a layer with an outer edge 8 microns below the surface
of the particles.
Table 2 shows the amounts of palladium and gold lost by the two
catalysts during the attrition test.
TABLE 2 Catalyst palladium loss gold loss A 56.0% 52.2% B 4.0%
5.9%
The results in table 2 show that the catalyst according to the
present invention loses less of the catalytically active metals
palladium and gold than the shell type catalyst.
* * * * *